1. Technical Field
The present invention relates to high strength hot rolled steel plates which are intended for use, for example, in automobile under-carriage components mainly produced by pressing, have a thickness of about 1.0 to 6.0 mm, have a strength of not less than 690 N/mm2, and possess excellent bore expandability and ductility, and a process for producing the same.
2. Background Art
In recent years, environmental problems posed by automobiles have lead to an increase in needs for a reduction in weight of car bodies and a reduction in cost by one-piece molding of components, for improving fuel consumption. To meet these needs, the development of high strength hot rolled steel plates possessing excellent press workability has been forwarded. Well-known conventional high strength hot rolled steel plates for such working include those having a composite structure, such as a ferrite-martensite structure or a ferrite-bainite structure, and those having a substantially single-phase structure composed mainly of bainite or ferrite.
In the ferrite-martensite structure, however, cracking occurs as a result of the formation of microvoids around martensite from an early stage of deformation, and, thus, the ferrite-martensite structure suffers from a problem of poor bore expandability. This renders steel plates having a ferrite-martensite structure unsuitable for use in applications, such as under-carriage components, where a high level of bore expandability is required.
In high strength hot rolled steel plates, it is known that bore expandability and ductility are likely to be contradictory to each other. Specifically, reducing the difference in hardness between ferrite and bainite is one means for improving the bore expandability in the ferrite-bainite structure. In this case, however, matching the hardness to that of hard bainite results in significantly deteriorated ductility, while matching the hardness to that of soft ferrite results in unsatisfactory strength. For compensation for the lack of strength, a large amount of precipitates should be dispersed to strengthen the steel plate. As a result, the ductility is lowered. Japanese Patent Laid-Open Nos. 88125/1992 and 180426/1991 disclose steel plates having a structure composed mainly of bainite. Due to the nature of the structure composed mainly of bainite, however, the amount of the soft ferrite phase is so small that the ductility is poor, although the bore expandability is excellent. Japanese Patent Laid-Open Nos. 172924/1994 and 11382/1995 disclose steel plates having a structure composed mainly of ferrite. These steel plates possess excellent bore expandability. Since, however, hard carbides are precipitated for ensuring strength, here again, the ductility is poor.
Japanese Patent Laid-Open No. 200351/1994 discloses a steel plate having a ferrite-bainite structure which possesses excellent bore expandability and ductility, and Japanese Patent Laid-Open No. 293910/1994 discloses a production process of a steel plate having a combination of good bore expandability with good ductility wherein two-stage cooling is adopted to regulate the proportion of ferrite. However, for example, a further reduction in weight of automobiles and increased complexity of components have led to a demand for a higher level of bore expandability and a higher level of ductility, and a high level of workability and a high level of strength, which cannot be satisfied by the above conventional techniques, are required of steel plates and sheets.